This invention relates to a data transmission system, and is described herein as applied to a full duplex 144 Kb/s subscriber loop transmission system.
The present invention relates to an improvement of the invention of British Published Appln. No. 2161676A.
That application describes a data transmission system in which the data to be transmitted is in digital form, and is transmitted over the line in an analogue form. To maintain synchronization between the ends of the system a pilot tone is sent in addition to the data, this pilot tone having an amplitude which is small compared with that of the data signal. The pilot tone frequency has a fixed and known relation to the data bit rate, e.g. one-half of that bit rate. Thus the addition of the pilot tone does not increase the bandwidth. Sampling of the data signal is effected under the control of the local clock, and adjustment of that local clock is effected in response to the detected pilot tone. This detection occurs concurrently with the sampling and detection of the data signals, and the local clock is compared with the received and detected pilot tone, the timing of the local clock being adjusted in accordance with the results of that comparison to maintain synchronism.